feat(enum): return enum from object keys

[ecyrbe]

This should close this issue: #1085

usage

const result: Z.ZodEnum< [ 'a', 'b' ] > = z.object({ a: z.string(), b: z.string().optional() }).enum();

it allows to extract the keys of a zob object schema as a zod enum.

Tell me if it's ok.

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[colinhacks]

That conversion from a union to a tuple is some black magic. Love it.

I renamed this method to keyof to stay in closer agreement with TypeScript terminology.

Great PR. 🙌

[ecyrbe]

@colinhacks yeah, i became a typescript vodoo master recently. Don't ask me what it cost me 🤣

[betafcc]

Really cool trick @ecyrbe, discovered half of it to make tuple permutations but didn't occur to me to use the function overload trick to get ordering, nice.

Do beware of how TS treats key order differently than JS engines though:

z.object({ a: z.string(), '1': z.string() }).keyof()._def.values
// TS says it's ['a', '1']
// JS runtime it's ['1', 'a']

Seems to not be an issue for internal use with zod enum, but thought pointing this out can be of some value since the test cases only have non numeral strings